When you type in your username, if you choose @gmail.com, you will be offered automatically setup, uncheck that, otherwise you get a POP connection, you probably want IMAP, so you have to do it manually.
choose POP or IMAP or Exchange (you have the additional setting of OWA in that case)
next, set up SMTP (note you cannot use our regular outgoing server, Macs and iPod/iTouch are both incompatible with it)
your mails should start to flow in.
you can go back and add all 3 kinds of accounts from the preferences menu
Next, you can set up address lookup using our Directory server